1.) Field of the Invention
The present invention relates to devices and methods for measuring the breath concentration of carbon monoxide and other substances.
2.) Background of the Invention
There are many pathological conditions for which it would be clinically useful to measure the rate of hemolysis, or degradation of red blood cells, occurring within the body. These conditions include neonatal jaundice, anemia and many others. Exhaled carbon monoxide can be used as an index of the rate of hemolysis because carbon monoxide is formed when hemoglobin, which is contained in the red blood cell, is released from the red blood cell and broken down by enzymes.
The degradation of hemoglobin results in the equimolar formation of bilirubin and carbon monoxide. After formation, the carbon monoxide binds to hemoglobin within circulating red blood cells forming carboxyhemoglobin. The carboxyhemoglobin dissociates in the lung releasing carbon monoxide into the exhaled breath. The concentration of carbon monoxide present in the alveoli of the lungs is directly related to the concentration of carboxyhemoglobin within the blood.
Carbon monoxide is also present in the inhaled air as well, and this exogenous carbon monoxide will bind to hemoglobin to form carboxyhemoglobin in the lungs. In a steady state condition the difference between the inhaled concentration of carbon monoxide and the end tidal concentration will reflect the rate of hemolysis occurring within the body.
In many patients, including newborns, collection of end-tidal breath samples is difficult because the patient is unable to voluntarily control their breathing patterns to produce an end tidal sample.
One method of measuring end tidal carbon monoxide concentration from a non-cooperative patient is direct acquisition of an end tidal sample using a syringe or other suitable device, followed by analysis of the sample with gas chromatography or mass spectroscopy. Using this method, the clinician would observe the subject""s chest movements to determine when end tidal breath was being exhaled. Thus, this method has the disadvantage of requiring a skilled clinician to take samples and to run the analysis.
Another method uses sensors to measure a subject""s breath carbon monoxide concentration. This method, which is described in U.S. Pat. No., 5,293,875, avoids the difficult task of inferring end tidal phases from chest movements, and avoids the need for gas chromatography or mass spectroscopy.
However, current electrochemical carbon monoxide sensors have relatively slow response times, so they cannot directly determine end tidal carbon monoxide concentrations. On the other hand, carbon dioxide sensors have relatively rapid response times, which allow for accurate measurements of end tidal carbon dioxide concentrations in normal-breathing humans. The ""875 patent addresses the shortcoming of the carbon monoxide sensors, by exploiting the fact that end tidal carbon monoxide concentration (corrected to exclude the ambient concentration), is proportional to end tidal carbon dioxide concentration. Since it is possible to determine end tidal carbon dioxide concentration, it is also possible to calculate end tidal carbon monoxide concentration, by multiplying mean carbon monoxide concentration by the ratio of end tidal carbon dioxide concentration to mean carbon dioxide concentration. The ""875 patent describes how to calculate end tidal carbon monoxide using this method.
A continuing challenge for the invention of the ""875 patent is presented by irregular breath patterns, which are especially common in infants. Irregular breath patterns can make it difficult to determine the beginning and end of an end tidal phase, and can cause disassociations between carbon monoxide and carbon dioxide measurements due to the different sensor response times, and due to variations in the depth (shallow or deep) or timing of the subject""s breathing.
The present invention marks an improvement over the method and apparatus of the ""875 patent. More particularly, the present invention allows for more robust and accurate determination of end tidal carbon monoxide, by (i) determining carbon monoxide concentration on a breath-by-breath basis, (ii) improving the algorithm that detects when an end tidal phase is over, and (iii) rejecting test data whenever breath or system variability is so great as to likely impede accurate testing.
The present invention is an improved method and device for the detection of end tidal carbon monoxide concentration. Under the present invention, two sensors are used to gather data about the concentration of gases in the subject""s exhaled breath. One sensor measures a first gas, preferably carbon dioxide, and the second measures carbon monoxide.
The present invention improves upon prior art end tidal carbon monoxide analyzers in three ways.
First, while the prior art only calculated an average end tidal carbon monoxide concentration over a period of time (time-based), the present invention calculates local or instantaneous end tidal carbon monoxide concentrations for each breath (breath-based). In order to calculate instantaneous end tidal carbon monoxide, the present invention uses a software filter that allows the carbon dioxide sensor to mimic the response characteristics of the carbon monoxide sensor.
Second, the improved device provides an end tidal carbon dioxide phase filter to avoid the premature determination that an end tidal phase has ended. The purpose of this filter is to help to avoid false peaks, which result from temporary spikes in a declining carbon dioxide concentration (inhale phase) that can be misinterpreted as the beginning of a new exhaled breath phase.
Third, the present invention provides a means to reject test data when excessive breath variability likely makes the test unreliable. Finally, the present invention provides a means to reject test data when system malfunctions and other problems cause excessive levels of system variability.
With these three improvements, the present invention provides a more robust and reliable evaluation of end tidal gas concentrations.
The present invention is especially useful for infants, who are prone to excessive levels of hemolysis and other conditions which result in high blood levels of carbon monoxide. However, the present invention can be used with subjects of all ages, and with many different exhaled gasses to diagnose or monitor a great variety of conditions.